Relationships between Lu–Hf and Sm–Nd isotopic systems in the global sedimentary system

ELSEVIER

Earth and Planetary Science Letters 168 (1999) 79–99

Relationships between Lu–Hf and Sm–Nd isotopic systems in the global sedimentary system Jeff D. Vervoort a,Ł , P. Jonathan Patchett a , Janne Blichert-Toft b , Francis Albare`de b a

Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA Normale Supe´rieure de Lyon, 46 Alle´e d’Italie, 69364 Lyon, Cedex 07, France

b Ecole

Received 20 May 1998; revised version received 20 November 1998; accepted 14 December 1998

Abstract We report new Hf (and Nd) data for more than 100 sedimentary samples, recent to Archean in age, from a wide range of depositional environments. These data document the behavior of Lu–Hf and Sm–Nd isotopic systems in the global sedimentary system. In conjunction with existing data for mantle-derived rocks, we now have reasonable constraints on coupled Hf–Nd isotopic behavior in the crust and mantle. Lu=Hf and Hf isotopic compositions are strongly fractionated between muds and sands in passive margin sediments due to concentration of low Lu=Hf, low 176 Hf=177 Hf, Hf-rich zircons in mature sands. In active margin settings, Lu–Hf fractionation due to the ‘zircon effect’ is minor due to the less weathered and more juvenile character of the sediments. Nd isotopic compositions are not highly fractionated by sedimentary sorting because heavy minerals, also rich in REEs, do not fractionate Sm–Nd efficiently. The lack of a large and systematic fractionation at active margins means that no significant Hf–Nd decoupling occurs here. This is important because sediments at active margins are the most likely to be recycled to the mantle. Hf–Nd isotopic data for all terrestrial samples fall along a single coherent trend ("Hf D 1:36"Nd C 2:95) which we call the ‘terrestrial array’. This array is composed of two complementary components: a mantle array ("Hf D 1:33"Nd C 3:19, defined by all oceanic basalts; and a crustal array ("Hf D 1:34"Nd C 2:82), defined by sediments, continental basalts, granitoids, and juvenile crustal rocks. The similarity of the crustal and mantle arrays indicates that no large-scale Hf–Nd decoupling occurs between the crust and mantle. The coherency of the terrestrial Hf–Nd array implies mixing within the mantle, due to stirring processes, and also within the crust, due to homogenization by collective sedimentary processes. In addition, tight Hf–Nd covariation may also imply that efficient crust to mantle recycling has modulated isotopic correlation in the silicate Earth. All Hf–Nd arrays, including the terrestrial array, lie significantly above (2–3 "Hf units) the BSE (bulk silicate Earth) reference. This would appear to require a hidden reservoir in the Earth, heretofore unsampled, to balance the Hf–Nd isotopic composition of the terrestrial array. However, the discrepancy between the terrestrial array and BSE may simply be due to differences in the way the CHUR (chondritic uniform reference) values were determined for the Lu–Hf and Sm–Nd isotope systems.  1999 Elsevier Science B.V. All rights reserved. Keywords: lutetium; hafnium; Sm-147=Nd-144; isotope ratios; sediments

Ł Corresponding

author. Fax: C1-520-621-2672; E-mail: [email protected]

0012-821X/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 9 9 ) 0 0 0 4 7 - 3

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1. Introduction It has long been recognized that Lu–Hf and Sm– Nd isotope systems behave analogously during most magmatic processes and that the Hf and Nd isotopic compositions should be broadly correlated in most crustal and mantle rocks (e.g., [1]). The degree to which this occurs in the mantle and crust, however, has not been well established due to the great potential for decoupling these isotopic systems in a number of important mantle (e.g., [2]) and crustal (e.g., [3,4]) environments. The reason we know little about the Lu–Hf isotopic composition of crustal materials is that until now there have been only scant data on continental crustal rocks and sediments. Here we report Lu–Hf isotopic data (and Sm–Nd data where needed) for over 100 sediment samples, recent to Archean in age, from a variety of depositional environments. These samples give us insights into the diversity of Hf–Nd behavior in the crust through time. We compare these new sediment data with existing data from the continental crust and oceanic basalts in order to determine the relationship between Hf and Nd in the crust and in the mantle. These relationships have important bearing on the broader issues of crust–mantle recycling, the efficiency of convection in the mantle, and the sedimentary homogenization of the crust.

2. Background and sample selection We have analyzed both modern sediments and ancient sedimentary rocks. The modern sediments include 6 pelagic clays and 27 modern deep-sea turbidites from a variety of tectonic environments [5]. The pelagic sediments are re-analyzed from a previous study by White et al. [6]. The turbidite suite had been previously analyzed for Nd and Sr isotopes, as well as major- and trace-elements in a comprehensive study by McLennan et al. [5]. The ancient sediments have been arbitrarily divided into Archean shales, post-Archean juvenile sediments, Paleozoic turbidites, and fluvial or shallow-water sediments. The Archean shales are from six different greenstone belts in the Canadian shield (Abitibi, Quetico, Wabigoon, and Sachigo subprovinces of the Superior Province; Slave Province;

Churchill Province) and from the Witwatersrand and Pongola basins in the Kaapvaal Craton [7]. The sediments from post-Archean juvenile terranes are from the following four geographic areas: Cretaceous to Cambrian greywackes, mudstones and sandstones from the Canadian Cordillera in southeast Alaska and western British Columbia [8]; Paleoproterozoic greywackes from northern Wisconsin [9] and central Sweden [3]; and Paleoproterozoic shales and pelites from the Birimian Terranes in west Africa [10]. The Paleozoic turbidite group consists of Carboniferous to Ordovician shales and sandstones from the Ouachita and Marathon assemblages and the Sevier–Martinsburg basin in the south-central U.S. [11], and Silurian shales and greywackes from Wales and Scotland [3]. The fluvial and shallow-water sediment grouping is made up of Devonian to Triassic shales and sandstones from Germany, Cambrian greywackes and shales from Britain and Belgium, a Mesoproterozoic sandstone from central Sweden [3], Paleoproterozoic slates and an arkose from the Marquette range supergroup in northern Michigan [9], and Paleoproterozoic shales from the ca. 2.2 Ga Huronian supergroup in southern Ontario [7].

3. Analytical techniques Analytical techniques for Lu–Hf separation chemistry and Lu–Hf isotope analysis have been presented in detail elsewhere [12–15] and will not be repeated here. All samples were dissolved in steel-jacketed Teflon dissolution vessels for 7 days at 160ºC as described in Vervoort and Patchett [13]. Lu–Hf chemical separation was done following the abbreviated 3-column procedure as described in Vervoort and Blichert-Toft [15]. All samples were analyzed for Lu and Hf isotopic composition using a total spike method in Lyon on a VG model Plasma 54 (P54) magnetic sector–inductively coupled plasma– mass spectrometer (MS–ICP–MS) [14,15]. All Lu and Hf isotopic analyses were carried out during two 8-day periods in 1997. During these 2 periods, 24 analyses of the JMC 475 Hf standard gave a 176 Hf=177 Hf value of 0:282161 š 14 (2¦ ). This range of values is a reasonable approximation of the external reproducibility of the samples reported here.

J.D. Vervoort et al. / Earth and Planetary Science Letters 168 (1999) 79–99

4. Results and discussion 4.1. Hf–Nd isotopic compositions and variations in sediments We have organized the sediment samples into four main groups for the purpose of presentation (Fig. 1; Table 1), i.e., muds (shales) and sands (sandstones, siltstones, greywackes), and whether they were deposited in an active or passive tectonic margin (e.g., [5]). The reasons for these broad divisions are to illustrate potential Hf–Nd fractionations between muds and sands [3] and highlight the differences between these two end-member tectonic classifications. These distinctions are important to delineate coupled Lu–Hf and Sm–Nd behavior in the crust and mantle. Muds are typically enriched in trace-elements (including Sm, Nd and Lu) due to their relatively high concentrations in clays [5] and therefore tend to have higher Lu=Hf ratios. Mature sands have higher concentrations of Hf due to the presence of zircon, but have overall lower concentrations of other traceelements because of the dilution effect of quartz and feldspar [3,5] and, therefore, have lower Lu=Hf ratios. Passive margin sediments tend to be derived from older and more compositionally evolved sources [5,16,17]. They are typically more fully weathered than active margin sediments and are composed of a high proportion of quartz grains [5,18]. A good portion of these sediments have probably been recycled from older sedimentary rocks [19]. Active margin sediments, on the other hand, tend to be derived from more juvenile material. These sediments are typically much less mature and are composed of a high proportion of rock fragments, many of which are volcanic in origin. Exceptions to this are sediments from some back-arc environments which may be derived from older cratonic source regions [5]. Passive margin sediments have a higher preservation potential than sediments from active margins. Active margin sediments are often deposited near consuming tectonic margins and are far more likely than passive margin sediments to be recycled into accretionary wedges, to be incorporated into magmatic products or mountain belts, or even be subducted into the mantle. Classifying modern sediments into passive or ac-

81

tive tectonic margin settings (Table 2) is fairly clearcut for most samples but is less obvious for sediments from hybrid tectonic environments or older sediments of uncertain tectonic setting. In the active margin classification we include back-arc, fore-arc, continental-arc, and strike-slip environments from modern sediments and, from the older sedimentary rocks, all the post-Archean sediments of juvenile terranes. In the passive margin classification we have only three modern turbidite samples (two trailing edge; one continental collision), but include all the Paleozoic turbidites from the Ouachita and the Caledonian fold belts as well as all samples from the ‘fluvial or shallow-water sediment’ group (Table 1). The Ouachita and Caledonian sandstones and shales we examined are not passive margin sediments in any geological sense, as they were deposited in response to Paleozoic orogenic events. However, these sediments are mature, have no juvenile arc components, and plot in the recycled orogen field of Dickinson et al. [19]. The Ouachita sandstones are interpreted to represent multi-cyclic sediments derived from the distant Appalachian Orogen [11]. Similar arguments can be made for the Caledonian sediments from Britain and northern Europe. We have decided to classify the Archean shales separately due to the well-documented differences from post-Archean shales [20]. We have also separated the six pelagic samples because of their obviously different depositional environment. Initial "Hf and "Nd values of all sediments were calculated at their depositional age and are shown in Fig. 1a. The sands and muds from active margins span a large range of Hf–Nd isotopic compositions (nearly 35 "Hf and 25 "Nd units) but are dominated by positive " values, verifying a strong juvenile component in these sediments. There are no systematic differences between the muds and sands in terms of their Hf–Nd isotopic composition. The Archean shales fall in the middle of this active margin field, with "Hf and "Nd values closer to chondritic. Taken together the active margin muds–sands and the Archean shales fall along a general trend of "Hf D 1:45 š 0:13"Nd C 3:19 š 0:81. The muds and sands from active margins have Sm=Nd and Lu=Hf ratios that are similar (e.g., mean 176 Lu=177 Hf: 0.015, muds; 0.015, sands) and plot in the same large area in Fig. 1b. The large range in parent=daughter ratios for

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Table 1 Sample descriptions Sample

Location

Sample type

Geological unit=location

Age (Ga)

Ref.

Pelagic sediments RC17-198 V21-196 (212) V25-5 (31–33) V14-55 (260) GS7605-55 V24-209 (45–48)

Pacific Ocean Pacific Ocean Atlantic Ocean Atlantic Ocean Atlantic Ocean Indian Ocean

red clay red clay brown clay siliceous ooze terrigenous silt terrigenous clay

pelagic sediment pelagic sediment pelagic sediment pelagic sediment pelagic sediment pelagic sediment

0 0 0 0 0 0

[6] [6] [6] [6] [6] [6]

Deep-sea turbidites V27-3-S (TE4-S) RC13-220-S (TE7-S) V29-20-S (CC10-S) RC10-84-S (CA24-S) RC10-87-S (CA25-S) RC10-87-M (CA25-M) RC9-81-S (CA27-S) RC15-58-S (CA28-S) V15-34-S (CA29-S) V15-34-M (CA29-M) V28-357-M (CA30-M) V28-357-S (CA30-S) V29-1-S (CA31-S) RC12-44-S (SS12-S) RC14-154-S (SS13-S) V28-327-S (BA16-S) V28-327-M (BA16-M) V24-131-S (BA18-S) RC14-123-S (BA19-S) RC14-132-S (BA20-S) V28-264-S (BA21-S) RC12-374-S (BA23-S) RC12-374-M (BA23-M) V28-257-S (FA32-S) V24-153-S (FA34-S) RC14-151-S (FA35-S) V28-283-S (FA36-S)

Sohm Abyssal Plain Angola Basin Ganges Cone West Mexico Basin Middle America Trench Middle America Trench Peru–Chile Slope Peru–Chile Trench Peru–Chile Trench Peru–Chile Trench Java Trench Java Trench Java Trench South Baja Basin Alaska Basin Celebes Basin Celebes Basin South China Basin Aleutians Basin Komandiorski Basin Japan Basin Japan Basin Japan Basin Marianas Basin North Solomon Basin South Aleutians Basin East Japan Basin

sand sand sand sand sand mud sand sand sand mud mud sand sand sand sand sand mud sand sand sand sand sand mud sand sand sand sand

trailing edge margin, Atlantic Ocean trailing edge margin, Atlantic Ocean continental collision basin, Indian Ocean continental arc basin, East Pacific Ocean continental arc basin, East Pacific Ocean continental arc basin, East Pacific Ocean continental arc basin, East Pacific Ocean continental arc basin, East Pacific Ocean continental arc basin, East Pacific Ocean continental arc basin, East Pacific Ocean continental arc basin, Indian Ocean continental arc basin, Indian Ocean continental arc basin, Indian Ocean strike-slip margin, East Pacific Ocean strike-slip margin, North Pacific Ocean back-arc basin, western Pacific Ocean back-arc basin, western Pacific Ocean back-arc basin, western Pacific Ocean back-arc basin, Bering Sea back-arc basin, North Pacific Ocean back-arc basin, western Pacific Ocean back-arc basin, western Pacific Ocean back-arc basin, western Pacific Ocean fore-arc basin, western Pacific Ocean fore-arc basin, western Pacific Ocean fore-arc basin, North Pacific Ocean fore-arc basin, western Pacific Ocean

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

[5] [5] [5] [5] [5] [5] [5] [5] [5] [5] [5] [5] [5] [5] [5] [5] [5] [5] [5] [5] [5] [5] [5] [5] [5] [5] [5]

River sediment AMA-AMAZ-M AMA-AMAZ-S

Brazil Brazil

mud sand

Amazon river sediment Amazon river sediment

0 0

Sediments from juvenile terranes (post-Archean) Canadian Cordillera 87MK-56 Alaska AT-86-49-1 British Columbia AT86-38-1 British Columbia AT85-117-1 British Columbia AT84-82-7 British Columbia AT86-123-2 British Columbia 87SP-22 Alaska 87MK-65 Alaska 87SP-29 Alaska 87-SP-21 Alaska

sandstone greywacke greywacke shale siltstone greywacke siltstone mudstone sandstone sandstone

Alexander terrane, Canadian Cordillera, Alaska, USA Stikine terrane, Canadian Cordillera, British Columbia, Canada Stikine terrane, Canadian Cordillera, British Columbia, Canada Stikine terrane, Canadian Cordillera, British Columbia, Canada Stikine terrane, Canadian Cordillera, British Columbia, Canada Stikine terrane, Canadian Cordillera, British Columbia, Canada Alexander terrane, Canadian Cordillera, Alaska, USA Alexander terrane, Canadian Cordillera, Alaska, USA Alexander terrane, Canadian Cordillera, Alaska, USA Alexander terrane, Canadian Cordillera, Alaska, USA

0.14 0.16 0.16 0.16 0.19 0.27 0.37 0.42 0.43 0.56

[8] [8] [8] [8] [8] [8] [8] [8] [8] [8]

Proterozoic sediments, Wisconsin W-413 Wisconsin W-410 Wisconsin 803-1 Michigan

schist schist schist

Wisconsin magmatic terrane, Wisconsin Wisconsin magmatic terrane, Wisconsin Marquette range supergroup, northern Michigan

1.88 1.88 1.85

[9] [9] [9]

Proterozoic sediments, Sweden SW-47 Sweden SW-48 Sweden

greywacke greywacke

Torringen schist, Bra¨cke, Sweden Torringen schist, Bra¨cke, Sweden

1.86 1.86

[3] [3]

M ODERN

A NCIENT

SEDIMENTS

SEDIMENTS

J.D. Vervoort et al. / Earth and Planetary Science Letters 168 (1999) 79–99

83

Table 1 (continued) Sample

Location

Sample type

Geological unit=location

Age (Ga)

Ref.

Birimian Terranes 40E39 AC36C G24 HL122

W. Africa W. Africa W. Africa W. Africa

schist black shale pelite schist

Kedougou–Kenieba inlier, Birimian Terranes, Mali, W. Africa Baoule–Mossi domain, Birimian Terranes, Ivory Coast, W. Africa Baoule–Mossi domain, Birimian Terranes, Guinea, W. Africa Kedougou–Kenieba inlier, Birimian Terranes, Mali, W. Africa

2.10 2.10 2.10 2.10

[10] [10] [10] [10]

Archean shales BW-S.1 AB-S.1 Q-S.1 W-2 W-3 WB-S.1 NSL-S.1 PAG-S.1 PO-6.2 PO-8 PO-10

Canada Canada Canada South Africa South Africa Ontario Ontario NW Canada South Africa South Africa South Africa

shale shale shale shale shale shale shale shale shale shale shale

Burwash Formation, Slave Province, Northwest Territories Abitibi subprovince, Superior Province, Quebec Quetico subprovince, Superior Province, Ontario Witwatersrand basin, Kaapvaal Craton Witwatersrand basin, Kaapvaal Craton Wabigoon subprovince, Superior Province, Ontario Sachigo subprovince, Superior Province, Ontario Prince Albert Group, Churchill Province, Northwest Territories Pongola supergroup, Kaapvaal Craton Pongola supergroup, Kaapvaal Craton Pongola supergroup, Kaapvaal Craton

2.66 2.70 2.70 2.72 2.72 2.80 2.80 2.80 2.90 2.90 2.90

[7] [7] [7] [7] [7] [7] [7] [7] [7] [7] [7]

Paleozoic turbidites OUA-10 Oklahoma OUA-11 Oklahoma MAR91-1 Texas MAR91-2 Texas OUA-8 Oklahoma OUA-9 Oklahoma OUA-12 Oklahoma OUA-13 Oklahoma OUA-22 Arkansas OUA-23 Arkansas OUA-41 Arkansas OUA-42 Arkansas OUA-17 Oklahoma OUA-18 Oklahoma OUA-35 Oklahoma OUA-36 Oklahoma OUA-29 Arkansas HOL-1 Tennessee HOL-2 Tennessee OUA-19 Oklahoma OUA-26 Arkansas OUA-25 Arkansas WAL-20A U.K. WAL-20B U.K. SU-19A U.K. SU-19B U.K. SU-16 U.K. SU-37A U.K. SU-37B U.K.

sandstone shale sandstone shale sandstone shale shale sandstone sandstone shale shale sandstone sandstone shale shale sandstone shale shale sandstone shale shale sandstone greywacke greywacke greywacke shale shale greywacke shale

Atoka Formation, Ouachita assemblage Atoka Formation, Ouachita assemblage Haymond Formation, Marathon assemblage Haymond Formation, Marathon assemblage Jackfork Formation, Ouachita assemblage Jackfork Formation, Ouachita assemblage Jackfork Formation, Ouachita assemblage Jackfork Formation, Ouachita assemblage Jackfork Formation, Ouachita assemblage Jackfork Formation, Ouachita assemblage Jackfork Formation, Ouachita assemblage Jackfork Formation, Ouachita assemblage Blaylock Formation, Ouachita assemblage Blaylock Formation, Ouachita assemblage Blaylock Formation, Ouachita assemblage Blaylock Formation, Ouachita assemblage Womble Formation, Ouachita assemblage Tellicco Formation, Sevier–Martinsburg basin Tellicco Formation, Sevier–Martinsburg basin Mazarn Formation, Ouachita assemblage Mazarn Formation, Ouachita assemblage Crystal Mountain Formation, Ouachita assemblage Aberystwyth grit, Aberystwyth, U.K. Aberystwyth grit, Aberystwyth, U.K. Tappins Group, Southern Uplands, U.K. Lowther beds, Southern Uplands, U.K. Lowther beds, Southern Uplands, U.K. Tappins Group, Southern Uplands, U.K. Tappins Group, Southern Uplands, U.K.

0.30 0.30 0.30 0.30 0.32 0.32 0.32 0.32 0.32 0.32 0.32 0.32 0.43 0.43 0.43 0.43 0.46 0.46 0.46 0.49 0.49 0.50 0.42 0.42 0.43 0.43 0.43 0.45 0.45

[11] [11] [11] [11] [11] [11] [11] [11] [11] [11] [11] [11] [11] [11] [11] [11] [11] [11] [11] [11] [11] [11] [3] [3] [3] [3] [3] [3] [3]

Fluvial or shallow-water sediments P-115 Germany P-118 Germany P-109 Germany P-111 Germany P-119 Germany DEV-1 Belgium ENG-12 U.K. DAL-3 Sweden P-146 Michigan P-149 Michigan P-151 Michigan P-153 Michigan MK-S.1 Ontario PEC-S.1 Ontario

red sandstone red shale red shale red sandstone quartzite quartzite greywacke red sandstone slate slate arkose slate shale shale

Buntsandstein, Marbach=Odenwald, F.R.G. Buntsandstein, Marbach=Odenwald, F.R.G. Ober-Rotliegendes, Bad Kreuznach, F.R.G. Unter-Rotliegendes, Bad Kreuznach, F.R.G. Taunusquarzit, Schlangenbad, F.R.G. Deville Formation, Ardennes, Belgium Bayston–Oakswood Group, Long Mynd, U.K. Dala sandstone, Klockara˚sen, Sweden Marquette range supergroup, northern Michigan Marquette range supergroup, northern Michigan Marquette range supergroup, northern Michigan Marquette range supergroup, northern Michigan McKim Formation, Huronian supergroup, Ontario Pecors Formation, Huronian supergroup, Ontario

0.21 0.21 0.25 0.26 0.39 0.55 0.55 1.33 1.85 1.85 1.85 1.85 2.20 2.20

[3] [3] [3] [3] [3] [3] [3] [3] [9] [9] [9] [9] [7] [7]

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J.D. Vervoort et al. / Earth and Planetary Science Letters 168 (1999) 79–99

Fig. 1. (a) Initial "Nd vs. "Hf and (b) 147 Sm=144 Nd vs. 176 Lu=177 Hf for all sediments reported in Table 2. Individual fields delineate muds and sands from active tectonic margins, passive margin muds, passive margin sands, and Archean shales (dashed border). Lu=Hf and Hf isotopic composition are fractionated between muds and sands in passive margins but not in active margins.

these sediments reflect differences between juvenile, immature muds and sands (high parent=daughter ratios) and more processed sediments that have experienced some mineral fractionation. However, Lu–Hf fractionation into low-Lu=Hf sands and high-Lu=Hf muds does not occur in any systematic way in these

sediments. This is illustrated by four sand samples that have higher Lu=Hf ratios than any of the muds. All of these four sands are derived from juvenile sources and have positive " values. Two of these are Paleoproterozoic sandstones from the Wisconsin magmatic terrane [9]. The other two samples

0 0

River sediment AMA-AMAZ-M AMA-AMAZ-S

Nd=144 Nd b

0.51193 0.51136 0.51132 0.51285 0.51235 0.51270 0.51247 0.51267 0.51247 0.51238 0.51193 0.51191 0.51271 0.51231 0.51272 0.51281 0.51285 0.51267 0.51236 0.51259 0.51243 0.51201 0.51216 0.51306 0.51298 0.51303 0.51267 š š š š š š š š š š š š š š š š š š š š š š š š š š š

3 3 3 2 3 2 3 2 2 2 2 2 3 2 2 4 2 2 2 2 2 3 2 2 2 2 3

0.512343 š 19 0.512392 š 18 0.511942 š 19 0.512553 š 20 0.511969 š 19 0.511886 š 15

143

Sediments from juvenile terranes (post-Archean) Canadian Cordillera 87MK-56 0.14 0.512683 š 7 AT-86-49-1 0.16 0.512699 š 4

SEDIMENTS

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Deep-sea turbidites V27-3-S (TE4-S) RC13-220-S (TE7-S) V29-20-S (CC10-S) RC10-84-S (CA24-S) RC10-87-S (CA25-S) RC10-87-M (CA25-M) RC9-81-S (CA27-S) RC15-58-S (CA28-S) V15-34-S (CA29-S) V15-34-M (CA29-M) V28-357-M (CA30-M) V28-357-S (CA30-S) V29-1-S (CA31-S) RC12-44-S (SS12-S) RC14-154-S (SS13-S) V28-327-S (BA16-S) V28-327-M (BA16-M) V24-131-S (BA18-S) RC14-123-S (BA19-S) RC14-132-S (BA20-S) V28-264-S (BA21-S) RC12-374-S (BA23-S) RC12-374-M (BA23-M) V28-257-S (FA32-S) V24-153-S (FA34-S) RC14-151-S (FA35-S) V28-283-S (FA36-S)

A NCIENT

0 0 0 0 0 0

SEDIMENTS

Age a (Ga)

Pelagic sediments RC17-198 V21-196 (212) V25-5 (31–33) V14-55 (260) GS7605-55 V24-209 (45–48)

M ODERN

Sample number

Table 2 Hf–Nd isotope data

0.1271 0.1345

0.132 0.108 0.096 0.130 0.117 0.129 0.108 0.131 0.106 0.125 0.100 0.115 0.131 0.116 0.129 0.140 0.153 0.141 0.102 0.121 0.104 0.112 0.113 0.197 0.168 0.146 0.105

4.04 2.30

5.94 1.63 2.54 3.12 3.46 5.55 3.52 4.24 4.96 4.89 6.38 5.69 5.40 3.55 3.69 2.95 3.51 3.14 2.80 3.97 3.24 3.56 6.12 1.22 2.30 4.41 3.33

2.32 5.89

0.1670 0.1140

Sm c (ppm)

10.85 35.34

Sm=144 Nd c

0.1370 0.1360

147

19.20 10.30

27.30 9.10 16.00 14.50 17.90 26.00 19.70 19.50 28.30 23.70 35.00 31.30 25.00 18.50 17.30 12.70 14.30 13.50 16.60 19.80 18.90 19.20 32.70 3.74 8.29 18.20 19.10

8.46 31.42

48.17 157.90

Nd c (ppm)

0.88 1.19

13.77 24.89 25.67 4.17 5.58 1.25 3.24 0.66 3.24 4.99 13.77 14.16 1.44 6.36 1.64 3.39 4.17 0.66 5.38 0.90 4.02 12.21 9.29 8.27 6.71 7.69 0.66

5.75 4.80 13.58 1.66 13.05 14.67

"Nd (0)

2.12 2.42

13.77 24.89 25.67 4.17 5.58 1.25 3.24 0.66 3.24 4.99 13.77 14.16 1.44 6.36 1.64 3.39 4.17 0.66 5.38 0.90 4.02 12.21 9.29 8.27 6.71 7.69 0.66

5.75 4.80 13.58 1.66 13.05 14.67

"Nd (T) Hf=177 Hf d

5 5 5 6 4 6

0.282932 š 7 0.282893 š 10

0.282301 š 3 0.282307 š 11

0.282080 š 4 0.281579 š 6 0.281471 š 7 0.283004 š 3 0.282855 š 8 0.282933 š 5 0.282738 š 4 0.282928 š 5 0.282471 š 6 0.282557 š 9 0.282312 š 3 0.282230 š 5 0.283033 š 6 0.282467 š 6 0.282922 š 5 0.283062 š 12 0.283108 š 11 0.282891 š 6 0.282570 š 4 0.282844 š 7 0.282828 š 7 0.282325 š 6 0.282664 š 10 0.283093 š 8 0.283153 š 7 0.283139 š 7 0.282966 š 5

0.282850 š 0.282901 š 0.282538 š 0.282960 š 0.282141 š 0.282486 š

176

Lu=177 Hf e

0.0122 0.0144

0.0079 0.0100

0.0025 0.0053 0.0048 0.0112 0.0079 0.0140 0.0105 0.0154 0.0081 0.0110 0.0114 0.0088 0.0155 0.0088 0.0109 0.0190 0.0193 0.0159 0.0095 0.0119 0.0149 0.0088 0.0146 0.0301 0.0295 0.0191 0.0139

0.0331 0.0656 0.0193 0.0228 0.0060 0.0135

176

0.37 0.21

0.50 0.08

0.63 0.08 0.17 0.27 0.25 0.40 0.29 0.36 0.37 0.31 0.41 0.35 0.54 0.32 0.30 0.39 0.39 0.27 0.27 0.30 0.39 0.19 0.32 0.25 0.34 0.51 0.35

0.77 2.04 0.51 0.26 0.54 0.34

Lu e (ppm)

4.25 2.05

9.01 1.11

35.43 2.12 5.11 3.44 4.42 4.03 3.89 3.26 6.50 3.96 5.09 5.58 4.99 5.10 3.95 2.88 2.89 2.36 0.70 3.62 3.67 3.13 3.14 1.20 1.65 3.76 3.58

3.30 4.40 3.75 1.63 12.81 3.46

Hf c (ppm)

5.66 4.28

16.66 16.44

24.47 42.19 46.01 8.20 2.94 5.69 1.20 5.52 10.64 7.60 16.27 19.17 9.23 10.79 5.30 10.26 11.88 4.21 7.14 2.55 1.98 15.81 3.82 11.35 13.47 12.98 6.86

2.76 4.56 8.28 6.65 22.31 10.11

"Hf f (0)

7.67 š 0.25 6.27 š 0.35

16.66 š 0.11 16.44 š 0.39

24.47 š 0.14 42.19 š 0.21 46.01 š 0.25 8.20 š 0.11 2.94 š 0.28 5.69 š 0.18 1.20 š 0.14 5.52 š 0.18 10.64 š 0.21 7.60 š 0.32 16.27 š 0.11 19.17 š 0.18 9.23 š 0.21 10.79 š 0.21 5.30 š 0.18 10.26 š 0.42 11.88 š 0.39 4.21 š 0.21 7.14 š 0.14 2.55 š 0.25 1.98 š 0.25 15.81 š 0.21 3.82 š 0.35 11.35 š 0.28 13.47 š 0.25 12.98 š 0.25 6.86 š 0.18

2.76 š 0.18 4.56 š 0.18 8.28 š 0.18 6.65 š 0.21 22.31 š 0.14 10.11 š 0.21

"Hf f (T)

J.D. Vervoort et al. / Earth and Planetary Science Letters 168 (1999) 79–99 85

0.511231 š 0.511013 š 0.511168 š 0.510713 š 0.510720 š 0.512357 š 0.511031 š 0.511154 š 0.511043 š 0.511009 š 0.510939 š 0.512074 š 0.512134 š 0.512011 š 0.512019 š 0.512072 š 0.512045 š 0.512023 š 0.512039 š 0.512088 š 0.512034 š 0.512018 š

2.66 2.70 2.70 2.72 2.72 2.80 2.80 2.80 2.90 2.90 2.90

Archean shales BW-S.1 AB-S.1 Q-S.1 W-2 W-3 WB-S.1 NSL-S.1 PAG-S.1 PO-6.2 PO-8 PO-10

Paleozoic turbidites OUA-10 0.30 OUA-11 0.30 MAR91-1 0.30 MAR91-2 0.30 OUA-8 0.32 OUA-9 0.32 OUA-12 0.32 OUA-13 0.32 OUA-22 0.32 OUA-23 0.32 OUA-41 0.32 6 6 7 6 8 7 6 7 5 6 7

7 6 7 8 7 5 6 8 6 7 7 0.1186 0.1379 0.0951 0.0986 0.1169 0.1218 0.1145 0.1230 0.1156 0.1170 0.1143

0.1117 0.1011 0.1157 0.0992 0.0985 0.1741 0.1049 0.1057 0.1165 0.1140 0.1109

0.1233 0.1272 0.1057 0.1047

0.511774 š 16 0.511751 š 5 0.511425 š 14 0.511485 š 26

2.10 2.10 2.10 2.10

Birimian Terranes 40E39 AC36C G24 HL122

0.1368 0.1585 0.1309 0.1489 0.1403 0.1048 0.1333 0.1315

Sm=144 Nd c

0.1086 0.1084

6 7 6 5 4 6 6 5

147

Proterozoic sediments, Sweden SW-47 1.86 0.511389 š 21 SW-48 1.86 0.511334 š 17

0.512682 š 0.512687 š 0.512769 š 0.512874 š 0.512614 š 0.512639 š 0.512608 š 0.512480 š

Nd=144 Nd b

0.1329 0.1567 0.1147

0.16 0.16 0.19 0.27 0.37 0.42 0.43 0.56

AT86-38-1 AT85-117-1 AT84-82-7 AT86-123-2 87SP-22 87MK-65 87SP-29 87-SP-21

143

Proterozoic sediments, Wisconsin W-413 1.88 0.511892 W-410 1.88 0.512251 803-1 1.85 0.511645

Age a (Ga)

Sample number

Table 2 (continued)

1.55 12.90 1.94 4.40 4.63 8.64 9.32 1.14 3.34 8.97 9.93

6.00 4.11 3.95 6.02 6.86 2.38 3.84 0.91 2.87 2.16 4.70

3.77 3.90 3.75 4.23

5.77 5.57

2.78 1.61 6.59

2.91 3.53 3.50 5.00 2.87 9.04 4.06 3.01

Sm c (ppm)

7.91 56.53 12.37 26.97 23.94 42.91 49.21 5.63 17.49 46.32 52.48

32.48 24.58 20.62 35.65 42.10 8.26 22.13 5.20 14.89 11.48 25.63

18.62 18.63 21.60 24.57

32.13 31.07

12.64 6.19 34.73

12.90 13.50 16.20 20.30 12.40 52.20 18.40 13.80

Nd c (ppm)

11.00 9.83 12.23 12.07 11.04 11.57 12.00 11.68 10.73 11.78 12.09

27.45 31.70 28.68 37.55 37.41 5.48 31.35 28.95 31.11 31.78 33.14

16.85 17.30 23.66 22.49

24.36 25.44

14.55 7.55 19.37

0.86 0.96 2.56 4.60 0.47 0.02 0.59 3.08

"Nd (0)

8.02 7.58 8.34 8.32 7.79 8.51 8.64 8.68 7.42 8.53 8.73

1.66 1.53 0.53 3.44 3.05 2.68 1.76 3.88 1.17 0.89 1.10

2.96 1.45 0.89 2.34

3.35 4.38

0.85 2.12 0.11

2.04 1.71 4.15 6.26 2.20 4.96 2.90 1.59

"Nd (T) Hf=177 Hf d

5 5 5 7

0.282109 š 0.282336 š 0.282163 š 0.282239 š 0.282205 š 0.282551 š 0.282380 š 0.282016 š 0.282214 š 0.282415 š 0.282476 š

6 4 7 3 4 5 7 5 5 4 7

0.281795 š 6 0.281438 š 6 0.281580 š 10 0.281405 š 5 0.281389 š 6 0.282254 š 7 0.281501 š 13 0.281261 š 5 0.281385 š 7 0.281473 š 8 0.281475 š 5

0.281988 š 0.281854 š 0.281747 š 0.281795 š

0.281714 š 10 0.281714 š 5

0.282720 š 21 0.282964 š 24 0.282136 š 7

0.282928 š 5 0.282962 š 5 0.282990 š 8 0.283033 š 10 0.282829 š 17 0.282845 š 8 0.282948 š 8 0.282771 š 14

176

Lu=177 Hf e

0.0032 0.0123 0.0039 0.0063 0.0041 0.0171 0.0147 0.0018 0.0040 0.0127 0.0168

0.0121 0.0063 0.0094 0.0091 0.0088 0.0209 0.0080 0.0021 0.0087 0.0093 0.0092

0.0103 0.0092 0.0066 0.0078

0.0076 0.0082

0.0275 0.0337 0.0128

0.0160 0.0230 0.0166 0.0215 0.0151 0.0152 0.0193 0.0176

176

0.08 0.56 0.19 0.44 0.32 0.53 0.69 0.17 0.23 0.59 0.70

0.32 0.18 0.23 0.56 0.56 0.26 0.15 0.04 0.22 0.16 0.32

0.30 0.42 0.19 0.25

0.33 0.30

0.21 0.23 0.43

0.27 0.44 0.28 0.43 0.29 0.38 0.33 0.30

Lu e (ppm)

3.40 6.39 7.05 9.95 11.12 4.39 6.72 13.31 8.37 6.65 5.88

3.71 4.09 3.43 8.68 9.08 1.78 2.65 2.63 0.35 2.48 4.95

4.06 6.48 4.11 4.46

6.09 5.16

1.11 0.95 4.76

2.39 2.73 2.42 2.83 2.73 3.53 2.44 2.42

Hf c (ppm)

23.45 15.42 21.54 18.85 20.05 7.82 13.86 26.74 19.73 12.63 10.47

34.55 47.18 42.15 48.34 48.91 18.32 44.95 53.44 49.05 45.94 45.87

27.73 32.46 36.25 34.55

37.42 37.42

1.84 6.79 22.49

5.52 6.72 7.71 9.23 2.02 2.58 6.22 0.04

"Hf f (0)

17.29 š 0.21 11.14 š 0.14 15.52 š 0.25 13.33 š 0.11 13.68 š 0.14 4.30 š 0.18 9.81 š 0.25 19.88 š 0.18 13.34 š 0.18 8.14 š 0.14 6.88 š 0.25

4.74 š 0.21 3.72 š 0.21 2.93 š 0.36 2.43 š 0.18 2.47 š 0.21 5.80 š 0.25 4.58 š 0.46 7.59 š 0.18 0.76 š 0.25 2.68 š 0.28 3.02 š 0.18

5.73 š 0.18 2.64 š 0.18 2.66 š 0.18 2.57 š 0.25

4.35 š 0.36 5.15 š 0.18

5.60 š 0.75 6.21 š 0.85 3.71 š 0.25

7.35 š 0.18 7.81 š 0.18 9.87 š 0.28 11.39 š 0.35 6.62 š 0.60 7.77 š 0.28 10.32 š 0.28 5.96 š 0.50

"Hf f (T)

86 J.D. Vervoort et al. / Earth and Planetary Science Letters 168 (1999) 79–99

Sm=144 Nd c

0.88 18.97 6.59 6.77 1.75 3.69 6.34 1.27 0.00 4.37 1.98 4.16 4.69 6.35

3.65 9.06 6.78 6.49 10.34 4.21 9.21 6.30 1.59 4.62 0.21 3.78 5.23 4.26 8.66 7.61 5.54 4.56

Sm c (ppm)

4.40 99.19 31.74 39.70 9.24 18.91 28.90 6.92 4.05 24.99 10.96 21.29 26.14 33.30

18.98 38.50 45.58 39.22 42.64 23.53 49.35 33.43 8.79 28.19 1.35 21.22 25.00 22.31 45.17 40.16 24.07 18.06

Nd c (ppm)

8.82 9.34 10.46 9.81 11.69 14.45 6.09 18.25 26.33 30.12 32.40 29.36 29.18 27.88

11.37 9.99 13.38 13.99 9.87 20.15 12.43 11.90 18.51 22.06 18.77 7.51 9.77 15.35 14.69 15.12 7.63 9.25

"Nd (0)

6.79 7.17 8.19 6.71 7.60 8.91 1.58 4.04 6.69 8.56 11.64 10.77 4.24 4.89

8.09 7.00 7.53 8.68 7.12 14.96 7.50 7.04 13.07 15.97 12.33 2.74 6.01 10.89 10.26 10.62 4.33 6.71

"Nd (T) Hf=177 Hf d

0.282186 š 6 0.282538 š 5 0.282475 š 9 0.282341 š 3 0.282459 š 6 0.281945 š 3 0.282561 š 5 0.281965 š 5 0.281767 š 10 0.281499 š 8 0.281185 š 6 0.281780 š 8 0.281559 š 6 0.281346 š 5

0.282166 š 12 0.282222 š 5 0.282595 š 10 0.282598 š 5 0.282222 š 11 0.281770 š 6 0.282448 š 3 0.282212 š 4 0.281980 š 16 0.281811 š 4 0.281548 š 14 0.282400 š 7 0.282447 š 6 0.282063 š 6 0.282347 š 5 0.282371 š 6 0.282503 š 6 0.282465 š 14

176

Lu=177 Hf e

0.0095 0.0199 0.0137 0.0053 0.0075 0.0043 0.0152 0.0085 0.0102 0.0077 0.0028 0.0119 0.0087 0.0044

0.0050 0.0177 0.0076 0.0187 0.0051 0.0059 0.0149 0.0052 0.0086 0.0076 0.0026 0.0088 0.0104 0.0065 0.0135 0.0136 0.0158 0.0158

176

0.06 0.82 0.43 0.34 0.08 0.23 0.48 0.15 0.29 0.07 0.06 0.30 0.29 0.49

0.29 1.09 0.24 0.59 0.61 0.28 0.63 0.57 0.09 0.19 0.01 0.38 0.39 0.27 0.51 0.45 0.49 0.38

Lu e (ppm)

0.95 5.85 4.41 9.25 1.42 7.53 4.50 2.42 4.05 1.27 3.07 3.53 4.81 16.00

8.17 8.75 4.49 4.47 16.81 6.73 6.01 15.44 1.46 3.52 0.58 6.09 5.38 5.96 5.40 4.71 4.37 3.44

Hf c (ppm)

20.72 8.28 10.50 15.24 11.07 29.25 7.46 28.54 35.54 45.02 56.12 35.08 42.90 50.43

21.43 19.45 6.26 6.15 19.45 35.43 11.46 19.80 28.01 33.98 43.29 13.16 11.49 25.07 15.03 14.18 9.51 10.86

"Hf f (0)

17.32 š 0.21 6.36 š 0.18 7.18 š 0.32 10.28 š 0.11 4.21 š 0.21 18.37 š 0.11 0.67 š 0.18 6.39 š 0.18 5.98 š 0.36 12.34 š 0.28 17.06 š 0.21 7.72 š 0.28 5.32 š 0.21 6.24 š 0.18

15.26 š 0.42 14.90 š 0.18 1.29 š 0.35 1.87 š 0.18 11.19 š 0.39 26.84 š 0.21 5.68 š 0.11 10.99 š 0.14 19.76 š 0.57 25.40 š 0.14 32.82 š 0.50 6.13 š 0.25 4.93 š 0.21 17.22 š 0.21 9.22 š 0.18 8.42 š 0.21 4.16 š 0.21 5.50 š 0.50

"Hf f (T)

b

For source of Sm–Nd data refer to Table 1. Most Nd data are taken from the references given in Table 1. For the samples analyzed at Arizona, 143 Nd=144 Nd ratios normalized to 146 Nd=144 Nd D 0.7219. LaJolla Nd standard gave 143 Nd=144 Nd D 0.511867 š 13 (2 standard deviations of the population; n D 15) during the course of this study. This compares to a 5-year average at the University of Arizona for LaJolla 143 Nd=144 Nd of 0.511869 š 15, n D 111. c 2¦ errors for Sm, Nd, and Hf concentrations and 147 Sm=144 Nd are <0.5%. d Ratios normalized to 179 Hf=177 Hf D 0.7325. e 2¦ errors for Lu concentrations and 176 Lu=177 Hf are <1.0%. f For calculation of "Hf values we used 176 Hf=177 HfCHUR(0) D 0.282772 and 176 Lu=177 HfCHUR(0) D 0.0332. Procedural blanks for Hf were <250 pg and for Lu were <1 pg. Sample to spike ratios for Hf were −1000.

a

0.1205 0.1156 0.1255 0.1031 0.1145 0.1178 0.1325 0.1113 0.1138 0.1058 0.1090 0.1182 0.1084 0.1153

147

Fluvial or shallow-water sediments P-115 0.21 0.512186 š 10 P-118 0.21 0.512159 š 6 P-109 0.25 0.512102 š 6 P-111 0.26 0.512135 š 6 P-119 0.39 0.512039 š 9 DEV-1 0.55 0.511897 š 12 ENG-12 0.55 0.512326 š 12 DAL-3 1.33 0.511702 š 12 P-146 1.85 0.511288 P-149 1.85 0.511094 P-151 1.85 0.510977 P-153 1.85 0.511133 MK-S.1 2.20 0.511142 š 6 PEC-S.1 2.20 0.511209 š 5

0.512055 š 0.512126 š 0.511952 š 0.511921 š 0.512132 š 0.511605 š 0.512001 š 0.512028 š 0.511689 š 0.511507 š 0.511676 š 0.512253 š 0.512137 š 0.511851 š 0.511885 š 0.511863 š 0.512247 š 0.512164 š

Nd=144 Nd b 0.1162 0.1422 0.0900 0.1000 0.1465 0.1081 0.1127 0.1139 0.1095 0.0991 0.0957 0.1078 0.1265 0.1153 0.1159 0.1145 0.1393 0.1525

0.32 0.43 0.43 0.43 0.43 0.46 0.46 0.46 0.49 0.49 0.50 0.42 0.42 0.43 0.43 0.43 0.45 0.45

OUA-42 OUA-17 OUA-18 OUA-35 OUA-36 OUA-29 HOL-1 HOL-2 OUA-19 OUA-26 OUA-25 WAL-20A WAL-20B SU-19A SU-19B SU-16 SU-37A SU-37B

143

5 5 6 7 6 5 8 5 5 7 7 5 5 6 7 8 5 6

Age a (Ga)

Sample number

Table (continued)

J.D. Vervoort et al. / Earth and Planetary Science Letters 168 (1999) 79–99 87

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J.D. Vervoort et al. / Earth and Planetary Science Letters 168 (1999) 79–99

are modern sands from near the North Solomon and Marianas Islands. Both modern sands are very immature with compositions dominated by volcanic lithic fragments, lesser feldspars, and no quartz [5]. It is apparent that these immature samples have not efficiently concentrated resistant minerals like zircon, and therefore significant fractionation of Lu–Hf between muds and sands has not occurred. The Archean shales have Sm=Nd and Lu=Hf values considerably lower (all 176 Lu=177 Hf  0.012, with the exception of one high Lu=Hf, Sm=Nd sample from the Wabigoon greenstone belt) than the juvenile sands and muds but have initial " values near zero and are in the same array as the muds and sands. In the case of these samples, the low Lu=Hf ratios most likely reflect low-Lu=Hf sources (tonalite–trondhjemite–granodiorite rocks, associated with LREE enrichment and HREE depletion) rather than any sedimentary fractionation. 4.2. Hf–Nd decoupling in the sedimentary system The sediments deposited in passive margin settings are fundamentally different from the juvenile, arc-dominated sediments from active margins. First, passive margin sediments generally have negative "Hf and "Nd values which reflect their derivation from older, cratonic sources. Second, the passive margin sands have compositions that are considerably different than the muds in terms of both initial Hf and Nd isotopes (Fig. 1a) and Lu=Hf and Sm=Nd ratios. The passive margin muds have Hf and Nd isotopic compositions that plot in the same array as the immature sediments and have Lu=Hf and Sm=Nd ratios generally lower than active margin muds and sands. The mature sands, however, are distinct from the muds in terms of both initial Hf isotopic compositions and Lu=Hf ratios. Most sands derived from evolved sources have lower initial "Hf values for a given "Nd value and plot along a much steeper trend ("Hf D 1:83"Nd C 0:60) toward more negative "Hf values. In terms of parent=daughter ratios, the passive margin sands have generally lower 176 Lu=177 Hf values than the muds (muds, 0.012; sands 0.007) but a similar range of 147 Sm=144 Nd values. The differences between muds and sands from active and passive tectonic environments are clearly illustrated (Fig. 2) by the isotopic data of mud–

sand turbidite pairs from two settings: (1) Paleozoic turbidites from the Ouachita–Marathon Fold Belt, south-central U.S. (passive margin association); and (2) modern turbidites from arc-dominated terranes (active margin association). The sands from Paleozoic turbidites have dramatically lower initial "Hf values (up to 17 "Hf units) but very similar "Nd values (all within 2 "Nd units) compared with corresponding mud layers from the same turbidite pair (Fig. 2). The modern turbidite pairs exhibit a much different behavior. The sand layers have lower initial "Hf values in all cases than the corresponding muds, but the differences are much less than in the Paleozoic turbidite couplets (4 of 5 are within 4 "Hf units). The difference in Hf–Nd behavior in modern turbidites is in Nd isotopes. The recent muds and sands from a single couplet typically have initial "Nd values that are much different (up to 7 "Nd units). The fractionation in parent=daughter ratios between muds and sands follows the fractionation of the initial "Hf and "Nd values, but only imperfectly. Most muds from the Ouachita turbidites have higher 176 Lu=177 Hf ratios than corresponding sands, consistent with their 176 Hf–177 Hf fractionation, but in one couplet this pattern is reversed with higher 176 Lu=177 Hf in the sand (Fig. 2b). Also, the 147 Sm=144 Nd ratios in the Ouachita turbidites are not fractionated between muds and sands in six of the couplets but are highly fractionated in three pairs, in spite of the fact that little 143 Nd–144 Nd fractionation occurs in any of the couplets. These differences in the fractionations of time-integrated parent=daughter ratios (i.e., initial " values) and present-day parent– daughter values between muds and sands reveal a complex interplay of sedimentary processes that are not completely linked. The primary process during the formation of turbidite mud–sand layers is the simple sorting of sediment into sand and silt–clay fractions. Zircons contain the majority of the Hf budget in a rock (about 1% Hf in zircon or 104 ppm) but much less Lu (usually 10 to 100 ppm) and other REEs. Concentration of zircons in sands will therefore result in a lowered Lu=Hf ratio but not in a major predictable change in Sm=Nd. Although zircons have high Sm=Nd ratios (¾0.4), the relative low abundance of zircons coupled with their moderate concentration levels of Sm and Nd (10 to 100 ppm) minimizes any potential

J.D. Vervoort et al. / Earth and Planetary Science Letters 168 (1999) 79–99

89

Fig. 2. (a) Initial "Nd vs. "Hf and (b) 147 Sm=144 Nd vs. 176 Lu=177 Hf for mud–sand turbidite pairs from the Paleozoic Ouachita Fold Belt (of recycled orogenic provenance; circles) and from recent active margin turbidites (squares). The Ouachita turbidites fractionate Lu=Hf and Hf isotopic composition due to the concentration of Hf-rich, low-Lu=Hf, low-176 Hf=177 Hf zircons, but not Sm=Nd or 143 Nd=144 Nd.

Sm–Nd fractionation due to zircon sorting. Thus for highly weathered samples that have been well sorted into clean, mature sands (e.g., quartz and zircon) and muds, we would expect significant fractionation of Lu–Hf but not Sm–Nd, which is precisely what occurs in the Ouachita turbidites.

Coupled with the Lu–Hf fractionation in the passive margin turbidites is a large time-integrated parent=daughter fractionation (initial " values). The reason this occurs in these samples is that (1) the mature sands have been derived from old sources where the zircon’s 176 Hf=177 Hf ratios evolved much

90

J.D. Vervoort et al. / Earth and Planetary Science Letters 168 (1999) 79–99

differently (owing to its very low Lu=Hf ratios) from the rest of the bulk rock, and (2) zircons tend to be concentrated in the sandstones. The fractionations in active margin settings, however, are significantly different. In modern active margin turbidites, the sands are typically less completely weathered as reflected in their higher proportion of lithics to quartz [5]. As a result there is little fractionation of Lu–Hf or initial " values between mud and sand owing to the juvenile source components and the immature nature of the sediment (e.g., BA-16). In some of these modern turbidite pairs (e.g., CA-25) there are complex fractionations that result in moderate changes in Sm=Nd and large fractionations in initial "Nd but these do not vary in any systematic way. McLennan et al. [5] suggested that the Nd isotopic fractionation in these arc settings is likely due to preferential breakdown of fine-grained (e.g., volcanic) and less stable components (e.g., glass, unstable minerals) and incorporation of these into the muds. These components are more likely to be material from recent volcanic arcs which would explain the much more juvenile signature (positive " values) in the muds.

finable mantle domain, it is becoming more clear that is not completely isolated from the rest of the mantle (e.g., [24,25]). When the MORB Hf–Nd isotopic data are viewed in the context of all ocean basalts, the MORB field lies on the radiogenic end of the OIB array with only somewhat larger range in Hf (Fig. 3). Hf–Nd isotopes are correlated in all ocean basalts and plot along a ‘mantle’ array (1:33 š 0:07"Nd C 3:19 š 0:52) that is within error of the OIB array (1:42 š 0:09"Nd C 2:57 š 0:51). There is certainly more Hf variation in MORBs relative to Nd than for the OIB array, but when viewed in the broader context of all oceanic basalt data, these Hf isotopic variations are less significant. Included in the oceanic array shown in Fig. 3 are all MORBs [2,21,22,26–33], OIBs [2,21,22,31,33– 46], and IAVs [2,47,48]. The island arc volcanic rocks (IAVs) plot along an array that has a somewhat shallower slope and higher intercept (IAVs, "Hf D 1:27 š 0:26"Nd C 4:87 š 1:63) than the OIBs, but are nonetheless coincident within error. The only oceanic basalts not included in this regression are the Cameroon Line basalts [49] which clearly plot off the main mantle array.

4.3. Hf–Nd isotopic variations in different crust and mantle reservoirs

4.3.2. Crust Hf–Nd isotopic data from continental regions exist for rocks of a variety of ages and geological contexts. In most cases, the number of samples is small. Two large data sets are recent, primarily mafic, volcanic rocks from the western U.S. [2,50–52], and a selection of Precambrian granitoids from diverse localities [4]. Not included in this compilation are early Archean gneisses from our work [15] or that of Barovich [53] because of potential Sm=Nd disturbances affecting these old gneisses [13]. Certainly this is not a data set representative of bulk continental crust, nor was it intended to be. However, it may reveal the range of Hf–Nd variations during magmatic processes. The Precambrian granitoids, in fact, were selected with the intent of finding anomalously high Hf=Nd ratios due to the presence of residual garnet in the source region for these granites. The mafic and felsic continental crustal samples are plotted with sediments and the oceanic basalt data in Fig. 4. Although there is significant scatter in these data sets and the mafic samples define an array shallower than that of OIBs [52], these data fall

In the context of the new sediment data presented above it is useful to review the existing Hf–Nd record for other samples of both continental and oceanic crust in order to get an overall picture of coupled Hf–Nd behavior in the crust and mantle. 4.3.1. Mantle It has been well known since the early 1980s that Hf–Nd isotopic compositions are well correlated in ocean island basalts (OIBs) but are not covariant in mid-ocean ridge basalts (MORBs) (e.g., [2,21–23]). The existing Pacific, Atlantic, and Indian MORB data plot as near-equidimensional fields and as a whole have a much larger spread in Hf (over 15 "Hf units) than Nd (6–7 "Hf units). This large Hf variation in MORB is due to melting of a depleted mantle source region with variable 176 Hf=177 Hf due to Lu–Hf fractionations from previous melting events [2,21,23]. Although the MORB source region appears to be a geochemically de-

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Fig. 3. Initial "Nd vs. "Hf for all oceanic basalts. Data taken from the following sources: MORBs [2,21,22,26–33], OIBs [2,21,22,31,33– 46], and IAVs [2,47,48].

Fig. 4. Initial "Nd vs. "Hf for mafic [2,50–52] and felsic [4] continental igneous samples and sediments (this study). For comparison are the field for OIBs and the trend for all terrestrial whole-rock data (see text for explanation of data set).

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within the fields for sediments and oceanic basalts on either ends of the Hf–Nd array. The Hf–Nd isotopic data for sediments (Fig. 1) plot along a diffuse array ("Hf D 1:67 š 0:13"Nd C 2:83š1:02) which corresponds to a slope steeper than the mantle array. The steeper slope of this array, and the greater dispersion at its unradiogenic end, however, are primarily due to mature sands from passive margins. As was demonstrated above, the lower "Hf values in these sands is due to the zircon effect. If we remove the sands from the regression and consider only the muds (Fig. 1a), these data plot along an array of "Hf D 1:44"Nd C 3:48, which is the same trend as all sediments excluding the recycled orogenic sands. For the purposes of estimating crustal Hf–Nd composition, the mud data are more appropriate because of the averaging process inherent in the formation and deposition of muds (e.g., [20]). The Lu=Hf ratios in the muds must, on average, be raised from average crust since the muds are a complement of the sands. However, it does not appear that the time-integrated Lu=Hf ratio of aver-

age mud is raised due to zircon removal in sands, as has been demonstrated for Mn-rich pelagic clays and Mn nodules [3,6,54]. Mature sandstones, in which the zircon effect is significant, are volumetrically far less significant than muds in the geological record (e.g., [20]). The deviations below the main terrestrial array toward unradiogenic Hf values, therefore, are most likely localized effects with little significance in Hf–Nd evolution of the bulk Earth (see also Plank and Langmuir [55]). 4.4. The terrestrial array and implications for mixing between crust and mantle We have combined all the terrestrial data in Fig. 5 and divided the data in a simplistic way into representative of either crust or mantle. The crustal samples include all the data discussed in the previous section with the exception of the three sands with extreme zircon effects, and the two Mn-rich red clays. All other samples are included. These data fall along an array ("Hf D 1:35 š 0:07"Nd C 2:82 š 0:43),

Fig. 5. Initial "Nd vs. "Hf for all terrestrial whole-rock samples in relation to bulk silicate Earth (BSE). All data fall along a single coherent array ("Hf D 1:36"Nd C 2:95) known as the ‘terrestrial array’, and is composed of two complementary arrays: the mantle array ("Hf D 1:33"Nd C 3:19) and the crustal array ("Hf D 1:35 C 2:82). All arrays pass above BSE as currently defined. See text for further explanation. Data sources as in Figs. 3 and 4.

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Fig. 6. Stratigraphic age vs. Nd- and Hf crustal residence ages, plotted in the manner of Goldstein et al. [16]. Model ages were calculated using linear " evolution from 0 to C10 for Nd and 0 to C16 for Hf, from 4.56 Ga to present. The similarity of the model age systematics underscores the overall coherent behavior of the Sm–Nd and Lu–Hf isotopic systems in the sedimentary environment.

which we here call the ‘crustal array’. All oceanic basalt data constitute the ‘mantle array’ ("Hf D 1:33 š 0:07"Nd C 3:19 š 0:52). This includes all OIB and MORB samples discussed in Section 4.3.1. Collectively, these data define what we will call the ‘terrestrial array’ which spreads along the line "Hf D 1:36 š 0:04"Nd C 2:95 š 0:26, and is nearly identical to both the mantle and crustal arrays. Considering the diversity of the samples it includes, this array is remarkably tight. It has a bow-tie shape, with smaller variations in Hf at the center (about 10 "Hf units), spreading out to nearly 15 "Hf units on the radiogenic end and over 15 "Hf units on the non-radiogenic end. The dispersion at the non-radio-

genic (crustal) end is due primarily to sedimentary fractionations involving zircon derived from older crustal sources. The dispersion at the mantle end of the array is due to previous Lu–Hf fractionations in the MORB source region [26]. The coherence of the Hf and Nd isotopic systems in sediments is underscored by the data in Fig. 6 which plots stratigraphic age vs. crustal residence age for Nd and Hf in the sediments presented here. For both Nd and Hf the age difference between the stratigraphic and crustal residence ages increase from 0–0.5 Ga at 3 b.y. ago to 0–2.7 Ga at the present. Both records seem to support models whereby the growth rates of the continents decrease through time

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(e.g., [16,56] from 4.0 b.y. ago to the present, perhaps reaching a quasi-steady-state in the Paleoproterozoic. The coherency of the terrestrial Hf–Nd array contrasts markedly with the Hf–Nd behavior in lunar basalts [57,58] where the data are vastly more heterogeneous and reflect at least two fundamentally different fractionation pathways. Low-Ti Apollo 12 basalts have extremely high initial "Hf (at 3.20 Ga), interpreted to reflect melting of a cumulate source, and plot on a Hf–Nd trend with an extreme slope of "Hf D 3:8"Nd C 7:0. Preservation of extreme Hf isotopic trends occur because in the lunar mantle there has been little or no convective mixing to destroy Lu–Hf fractionations formed early in its history. Convection in the terrestrial mantle is probably inevitable at all times in Earth’s history, but the manner in which it operated in the early Archean is uncertain. Nevertheless, the coherence of the terrestrial array argues that efficient mixing in the terrestrial mantle, particularly in the early Earth, has homogenized any major effects of early melting or crystallization-related fractionations. One possible interpretation of the data is that Lu–Hf and Sm–Nd isotope systems fractionate in a coherent fashion in the crust and the mantle. Whenever magmas are produced and separated from their sources, whether it be in the source region of MORBs, OIBs, continental basalts, or granites melted from pre-existing crust [4,59], Lu–Hf and Sm–Nd fractionations tend to be covariant, and Hf– Nd evolution stays within the terrestrial array. This holds for the sedimentary system as well. What type of fractionations occur, whether they involve Lu–Hf and Hf isotopic changes due to the concentration of zircon or some other effect, these differences are homogenized by collective sedimentary processes. This is particularly true in the formation of muds and shales, the dominant sediment delivered to ocean basins. A quite different interpretation is that fractionations in the mantle are different from those in the crust, but that recycling processes have obscured any major differences that may have been produced. If turbiditic sediments are recycled into the mantle with some pelagic mud component, these sediments may ‘flavor’ the mantle with an average crustal Lu=Hf and Sm=Nd character [3,55]. Differences in Lu–Hf

and Sm–Nd fractionations in the crust and mantle, therefore, may be buffered by efficient mixing within these reservoirs and also by the bi-directional exchange of material between the crust and mantle. 4.5. Bulk silicate Earth and the chondritic Lu and Hf values Although Hf–Nd data for nearly all terrestrial samples form a coherent array, one obvious feature of these data that needs to be addressed is the position of the terrestrial array relative to BSE (bulk silicate Earth). Although the discrepancy between the terrestrial array and BSE does not appear to be very large in Fig. 5, all best-fit arrays calculated for any subsets of data (Figs. 1 and 3–5) consistently lie about 3 "Hf units above BSE. Positive "Hf intercepts have been recognized as features of Hf–Nd arrays for some time [4,13,50] but until recently the magnitude of the intercept was sufficiently small (<2 "Hf units) in relation to the spread of the data and the uncertainty in the chondritic Lu–Hf values (which translated to about 2 "Hf units uncertainty) to not warrant concern. Recent, more precise determinations of the chondritic Lu–Hf values [60], however, have exacerbated this problem because the new chondritic Lu–Hf values result in "Hf values that are, on average, almost two "Hf units higher than the previous values [61] and cause the terrestrial Hf–Nd array to pass further above BSE [60]. Furthermore, we now have sufficient coverage of the crust and mantle that we can say with more confidence that we have sampled most of the major crust and mantle reservoirs. The consistency of the Hf–Nd data is illustrated in Fig. 7, which plots slope vs. intercept and their associated errors for all the data sets discussed in the preceding sections. The regression lines plotted here were calculated using the weighted leastsquare method. Although there are some differences between the individual regressions (most notably higher intercepts for the continental groups and IAVs), all are characterized by intercepts greater than 2 "Hf units. This demonstrates that all of the arrays, and notably the aggregate terrestrial array, pass significantly above the bulk silicate Earth reference point and indicate a fundamental mismatch between all terrestrial data and the chondritic reference. There

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Fig. 7. Plot of slope vs. intercept for various "Hf –"Nd arrays described in the text. The slope and intercept values, with their error envelopes, were calculated by a weighted least-squares method. These values differ somewhat from unweighted linear regression in Figs. 1–5, due to differences between the two methods, but the interpretation is the same for both. Regressions for all data sets are characterized by similar slopes and by intercepts consistently greater than 2 "Hf units. ‘All sediments ROS’ refers to the data set for all sediments excluding recycled orogenic sands. ‘All’ refers to the aggregate terrestrial array.

are three possible solutions to this problem: (1) there is a hidden reservoir in the Earth, heretofore unsampled, that balances known terrestrial Hf–Nd isotopic compositions; (2) the chondritic reference values for the Sm–Nd and Lu–Hf isotopic systems, either individually or collectively, do not accurately represent a bulk silicate Earth composition; or (3) the silicate Earth is not chondritic with respect to Hf and Nd. We choose to examine the first two of these possible solutions here and defer discussion of the more highly speculative non-chondritic silicate Earth until further warranted. If the presently used Hf and Nd chondritic values [60,62] accurately reflect the composition of the bulk silicate Earth, then a hidden reservoir would be required to balance the existing crust and mantle Hf–Nd data [60]. What this hidden reservoir would be, however, is speculative. Hiding this unradiogenic Hf reservoir in the crust is unsatisfactory because we have little evidence for such a negative "Nd –"Hf reservoir from the sediment and granitoid data. Some mature sands have the appropriate Hf–Nd composition (i.e., below the array in the negative "Nd –"Hf

direction) but are volumetrically much too small to balance the rest of the crust and mantle. If such a complementary, low-Lu=Hf reservoir does exist, its formation must have started very early in the history of the Earth and the material sequestered to some place, possibly in the deep mantle, where it is not sampled by any later magmas. Blichert-Toft and Albare`de [60] argued that perovskite fractionation [63–65] would produce a low-Sm=Nd, high-Lu=Hf residual mantle that would evolve toward the negative "Nd –positive "Hf quadrant. Much later melts from the mantle remaining after perovskite removal would have raised "Hf compared to "Nd , and thus would correspond to the present mantle array. The implausible aspect of this model, however, is that the complementary perovskite-rich reservoir would have to remain sequestered and unsampled through all of geologic time. Partition data for Lu, Hf, Sm, and Nd in pyrope garnet [64,66] predict that melts leaving garnet in the residuum would have low Sm=Nd and very low Lu=Hf and would evolve toward negative "Nd –"Hf

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values below the mantle array [60]. Blichert-Toft and Albare`de [60] suggested that melts such as these formed early in the history of the Earth, crystallized at the surface, and then were subducted to an inaccessible part of the mantle as suggested by Chase and Patchett [67]. This ancient, oceanic-type crust would have the proper negative "Nd –"Hf character needed for the missing component and the residual mantle left from this melt extraction would have super-chondritic Sm=Nd and Lu=Hf ratios consistent with the positive "Nd and "Hf values characteristic of the depleted mantle. Some partitioning data for majorite garnet, however, predict smaller Lu–Hf fractionations than for pyrope garnet [64,68–70] and therefore this phase may not have a significant decoupling effect on Hf–Nd systematics in the mantle [70]. Whatever model is proposed to explain the position of BSE below the terrestrial array must be reconciled with an important fundamental feature of the Hf–Nd data: none of the Hf–Nd data from oceanic basalts, including OIBs, fall sufficiently below the array in the negative "Nd –"Hf quadrant to suggest it had been derived from a missing unradiogenic Hf reservoir. If such an enriched primordial mafic crust did exist, it seems unlikely that it could be sequestered to some region in the mantle for the whole of Earth’s history with little evidence for its existence from mantle-derived magmas, especially considering that many OIBs are believed to have been produced in part from melting of such a source (e.g., [71]). An alternative, and perhaps more straightforward, explanation for the Hf–Nd–BSE paradox is that the chondritic reference does not accurately represent bulk silicate Earth. Salters and White [33] recently examined the mismatch of the OIB data with the chondritic reference. They suggest that this problem can be avoided by shifting the bulk Earth Hf and Nd reference into the center of the OIB array along a chondrite regression line [33]. Using this method they propose new present-day chondritic values of 176 Hf=177 Hf D 0.2828, 176 Lu=177 Hf D 0.0335, 143 Nd=144 Nd D 0.51259 and 147 Sm=144 Nd D 0.1952. With the addition of the sediment data presented in this paper and the recognition of a coherent Hf–Nd array for essentially all terrestrial samples, we concur that the Hf–Nd chondritic values need re-examination.

The discrepancy between the terrestrial array and BSE may simply be due to differences in the way the CHUR (chondritic uniform reference) values were determined for the Lu–Hf and Sm–Nd isotope systems. As pointed out by Jacobsen and Wasserburg [62] for the Sm–Nd isotope system, there is no way to uniquely determine a single solar system initial isotopic composition from the range of compositions present in chondrites. Jacobsen and Wasserburg [62] somewhat arbitrarily selected 143 Nd=144 Nd and 147 Sm=144 Nd ratios close to the values for the C2 and C3 carbonaceous chondrites, Murchison and Allende. These CHUR values are self-consistent with all chondritic data, but are at the upper end of the array in terms of both 143 Nd=144 Nd and 147 Sm=144 Nd [62]. Patchett [61] used this same approach and chose the 176 Lu=177 Hf values of Murchison to define the parent=daughter ratio of the chondritic uniform reference (CHUR) in order to achieve comparability between the Sm–Nd and Lu–Hf isotope systems. However, the 176 Hf=177 Hf value for Murchison measured by Patchett and coworkers was too imprecise to use due to multiplier TIMS measurements, and a CHUR 176 Hf=177 Hf value had to be determined from the initial isotopic composition of eucrite meteorites at 4.55 Ga [61]. This gave a present-day CHUR 176 Hf=177 Hf value of 0.282830 [61], adjusted for Hf standard values of more recent publications [4]. Blichert-Toft and Albare`de [60] measured more accurate 176 Hf=177 Hf values for 25 C, O, and E chondrites using multi-collector ICP–MS. They determined a present-day 176 Hf=177 Hf value of 0.282772 for CHUR using a statistical weighted average of all chondrites they measured. Whereas the chondritic reference determined in this way may be more valid than the approach for the Nd isotopic system [62], the two sets of values were not determined in the same way. It now seems to us that the best approach would be to use Murchison and Allende chondrites to determine the CHUR values for both the Lu–Hf and Sm–Nd isotope systems. The Allende and Murchison chondrites measured by Blichert-Toft and Albare`de have 176 Hf=177 Hf values above the statistical average for all chondrites reported [60]. Although there is a large range in the 176 Hf=177 Hf values for Allende (0:282775 š 8 to 0:282825 š 11) and

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Murchison (0:282899 š 35) the current accepted present-day chondritic value (0.282772) is at the lower end of this range. An unweighted average of these three measurements yields a 176 Hf=177 Hf value of 0.282833 which is essentially the same as the original present-day CHUR value suggested by Patchett [61]. Murchison and Allende chondrites, therefore, are consistent with a higher value of 176 Hf=177 Hf for the chondritic reference. If the present-day chondritic reference were to be set 2 "Hf units higher, due to an increase in present-day 176 Hf=177 Hf for CHUR of ¾0.00006 (and assuming no change in presentday 176 Lu=177 Hf, which we did not examine), then the bulk silicate Earth point would lie in the center of the terrestrial array and obviate any need for a hidden reservoir to balance the Hf–Nd isotopic compositions of known crust and mantle reservoirs.

5. Concluding remarks The overwhelming first-order observation from the Hf–Nd data presented and reviewed here is that nearly all terrestrial compositions plot along a single, reasonably coherent, array. Although there are variations within the array due to Lu–Hf and Sm–Nd fractionations in certain crust and mantle domains, none of these result in large-scale decoupling of Hf and Nd isotope systems. The coherency of the Hf–Nd array may imply efficient first-order mixing within the mantle, due to stirring processes, and separately within the crust, due to homogenization via the sedimentary system and crustal melting. On the other hand, tight Hf–Nd covariation may also argue for efficient crust to mantle recycling, modulating isotopic correlation in the silicate Earth. The second-order observation is that in spite of the overall coherence of Hf and Nd isotopic compositions on Earth, there is great heterogeneity within the terrestrial array. These complexities, with the terrestrial array as a backdrop, offer great potential for identifying processes in the crust and mantle that fractionate Hf and Nd isotope systems in subtle ways, and will be the source of important Hf–Nd isotopic studies in the future. Although the terrestrial array lies significantly above the BSE, based on our currently used Hf– Nd chondritic reference, we simply do not know

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those reference values well enough to determine whether or not this mismatch is real. Therefore, before we develop models based on any present offset between the terrestrial array and the BSE reference, it is imperative that we refine the reference Hf–Nd isotopic values.

Acknowledgements We are grateful to Scott McLennan and Sidney Hemming for providing the modern turbidite samples, Anne Marie Aucour for the Amazon River samples, and Jamie Gleason and Bill White for the Ouachita and modern pelagic sediments, respectively. Other samples from Patchett et al. [3] were given by Keith O’Nions, Go¨ran Bylund and Borwin Grauert. Scott Samson donated the Alexander and Stikine samples. Catherine Chauvel and Bill White generously allowed us to use their unpublished Hf and Nd data in our compilations and plots. We thank Scott McLennan and Catherine Chauvel for their thorough and constructive reviews. We are grateful to Philippe Telouk for his expertise in maintaining the P54 in top running condition. This research was funded by NSF grant EAR-9526536 to Patchett and Vervoort, including a supplement that was instrumental in enabling the collaboration between Arizona and Lyon. [CL]

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